Advantages of interstitial implantation of radiation-emitting material for localized tumor treatment has been recognized for some time now. Interstitially implanted materials concentrate the radiation at the place where this treatment is needed, i.e., within a tumor so as to directly affect surrounding tumor tissue, while at the same time exposing normal tissue to far less radiation than does radiation that is beamed into the body from an external source.
One early implantable radioactive material was gold wire fragments enriched in radiation-emitting gold isotopes, such as gold-198. An advantage of gold wire, for interstitial implantation is that gold is compatible with the body in that it does not degrade or dissolve within the body. Another commonly used implantable material is radon-222.
Materials, such as gold-198 and radon-222, have significant counterindicating characteristics for interstitial tumor treatment in that they emit relatively penetrating radiation, such as X-rays or gamma radiation of higher energy than is preferred, beta particles or alpha particles. Such materials not only subject the patient's normal tissue to more destructive radiation than is desired but expose medical personnel and other persons coming into contact with the patient to significant doses of potentially harmful radiation.
U.S. Pat. No. 3,351,049 describes capsules or seeds in which an enclosed outer shell encases an X-ray-emitting isotope having a selected radiation spectrum. Notably, the capsules contain iodine-125 having a radiation spectrum which is quite favorable for interstitial use compared to previously used materials. The encasing shell localizes the radioactive iodine to the tumor treatment site, preventing the migration of iodine to other parts of the body, notably the thyroid, which would occur if bare iodine were directly placed in the tumor site. The use of an encasing shell permits the use of other X-ray-emitting isotopes which would dissolve in the body or present a toxic hazard to the recipient. Capsules or seeds containing iodine-125 have been used in treating patients for some time now, and their general effectiveness has been described in several publications, for example, The Use of Iodine-125 for Interstatial Implants, U.S. Department of Health, Education, and Welfare Publication (FDA) 76-8022, Basil H. Hilaris, et al., November 1975.
Other isotopes have been suggested as alternatives to iodine-125. The '049 patent, in addition to iodine-125, suggests palladium-103 and cesium-131 as alternatives. Palladium-103 has the advantage of being an almost pure X-ray emitter of about 20-23 keV. Furthermore, it is compatible with the body in that it is substantially insoluble in the body. Thus palladium presents less of a potential hazard to the body, in the rare event of shell leakage, than does radioactive iodine, which if it were to leak from its encasing shell, would migrate to and accumulate in the thyroid with potentially damaging results.
Although the '049 patent suggests the use of seeds containing palladium-103, to date, only seeds containing iodine-125 have been commercially available. The reason that palladium-103 has not been used as an interstitial X-ray source is suggested in Medical Physics Monograph No. 7, "Recent Advances in Brachytherapy Physics", D. R. Shearer, ed., publication of the American Association of Physicists in Medicine, (1979) at page 19 where it is noted that its 17-day half-life (as compared with iodine-125 with about a 60-day half-life) is "just too short".
Indeed a 17-day half-life is difficult to work with in making capsules as produced according to the teachings of '049 patent in which substantially pure palladium-103 is contemplated. The short half-life represents a substantial obstacle to providing implants that contain substantially pure palladium-103. To produce substantially pure palladium-103, a transmutable element, such as rhodium-103, is converted to palladium-103 in a nuclear particle accelerator, and the palladium-103 is then isolated from untransmuted source material. The processing time of isolating the palladium-103 and additional processing time needed for encapsulating the radioactive material results in a substantial loss of activity of the palladium-103 before it is ever used in the body. Furthermore, producing palladium-103 by means of an atomic particle accelerator is difficult, and palladium-103 produced in this manner is very expensive. These considerations undoubtedly account for the fact that palladium-103 has not been incorporated in commercially available tumor treatment materials.
It is desirable to be able to use palladium-103 as an interstitially implantable X-ray source as the radiation spectrum of palladium-103 is somewhat more favorable relative to that of iodine-125. More importantly, the shorter half-life of palladium-103 relative to iodine-125, although presenting problems with respect to delivering the material to the patient, has important advantages with respect to patient care. The patient is significantly radioactive for a substantially shorter period of time and therefore poses less of a hazard to medical personnel and others who come in contact with the patient for the same period of time. By using a short half-life isotope for interstitial implantation, the time during which precautions against radiation exposure must be taken when treating the patient may be reduced, and the patient's periods of confinement in the hospital may be correspondingly reduced. As noted above, palladium does not present the potential problem of leaking iodine. Thus, it would be desirable to have methods and materials for making palladium-103 generally available as an implantable X-ray source.
A disadvantage of I-125-containing seeds, as presently produced, is that the seeds are anisotropic in their angular radiation distribution. This is due to the configuration of the capsules or seeds which are tubular and which, due to currently used shell-forming techniques, have large beads of encapsulating shell material at the sealed ends of the tubular structure. Although the '049 patent proposes unitary tubes that are sealed so as to have ends formed to be of substantially the same thickness as the sidewall of the tubular structure, the capsules actually produced by the assigness of the '049 patent have heavy beads of shell material at the ends of the seeds that result from the welding process. Such beads of material substantially shield emitted radiation, whereby the amount of radiation emitted from the ends of the capsule is substantially reduced relative to the amount of radiation emitted from the sidewall of the capsule. It would be desirable to produce implantable X-ray-emitting seeds with a more isotropic radiation distribution.